1. Field of the Invention
The present invention relates to a microporous thermal insulation molding comprising compressed thermal insulation material comprising 30-100% by weight of finely divided metal oxide, 0-50% by weight of opacifier, 0-50% by weight of fiber material and 0-15% by weight of inorganic binder.
2. The Prior Art
U.S. Pat. No. 4,985,163 discloses microporous thermal insulation moldings.
The finely divided metal oxides preferably used in such microporous thermal insulation moldings are pyrogenic silicas including electric arc silicas, precipitated silicas or silicon dioxide aerogels and also analogously prepared aluminum oxides. To achieve good thermal insulation properties these finely divided oxides possess very high specific surface areas which preferably lie in the range of 50-700 m.sup.2 /g (measured by BET, as described in ASTM Special Technical Publication No. 51, p. 1941 ff.). As a result of these large surface areas, these oxides have a very strongly pronounced ability to adsorb polar substances. As is known, they therefore act as very effective drying agents and in the natural atmosphere readily absorb water. Accordingly, the same behavior is shown by the microporous thermal insulation moldings produced therefrom. In practice, therefore, moisture absorption by the microporous heat insulation moldings during storage and use cannot be prevented. If in practical use these microporous thermal insulation moldings are now exposed to a high temperature within a short period of time, there is explosive formation of steam which destroys the structure of the thermal insulation molding. This effect occurs, for example, in microporous thermal insulation moldings used as thermal insulation in radiation heating units for ceramic hotplates, if these radiation heating units are brought to red heat by means of the customary resistance heating elements at the intervals of preferably from 1 to 5 seconds which are now customary for the newest generation of such units. However, these short heating times are unavoidable because of the changed consumer attitude.